As a rule, a flexible sensor mat filled with fluid is used with such a passive occupant recognition system to detect the pressure which a weight loading the respective vehicle seat brings about. The sensor mat is arranged beneath the seat foam and consequently at least substantially exposed to the same temperature as the seat foam. Since the pressure measured is dependent on the temperature to a high degree, as a rule a temperature sensor is used to measure the temperature of the system. The temperature sensor is preferably arranged within the electronic control and/or evaluation unit. The physically relevant temperature is, however, the foam temperature.
The temperature value measured by the temperature sensor has previously been used for the calculation of the temperature compensation value. On the heating up and cooling down of the vehicle or with all transitional temperature effects, the temperature of the electronic control and/or evaluation unit, however, deviates greatly from the foam temperature. The time constant decisive for the time behavior of temperature is in the range of approximately 30 min. for foam and at approximately 5 min. for the electronic control and/or evaluation unit.
In particular with greater temperature changes, a difference therefore occurs between the temperature measured by the temperature sensor and the temperature actually prevailing in the seat foam. The seat foam heats up more slowly than the temperature sensor or the region in which the temperature sensor is arranged. If now, for example, a decision is made with reference to the pressure measured whether an airbag should be switched on or not, such parameter values as, for example, the empty pressure and the allow threshold have to be changed in accordance with the temperature measured on the temperature compensation. In the case of greater temperature changes, an incorrect compensation or an incorrect setting of the parameter values in question therefore takes place in the present case based on the temperature measured by the temperature sensor.